فصلنامه رادار
سال نهم شماره 2 (پیاپی 26، پاییز و زمستان 1400)

n this paper, transmit beamforming and T/R modules' allocated power in active phased array radars are ‎optimized to reduce interception probability of radar under uncertainties in the interceptor’s direction. The LPI ‎problem is solved analytically and some theorems are extracted for optimum beamforming weights design. Two ‎LPI optimization problems from two perspectives are formulated and solved analytically in preparing two closed ‎form solutions in combination with heuristic iterative algorithms for designing transmit beamforming weights. ‎LPI performance and computation time of proposed algorithms are compared with the benchmark numerical ‎Barrier algorithm and with the conventional radar in which maximizing detection probability without ‎interception probability issues is considered. Simulation results show that proposed algorithms not only minimize ‎the interception probability of radar but also satisfy desired detection performance and power budget ‎constraints, simultaneously. The proposed algorithms outperform standard numerical barrier algorithm in ‎complexity and optimality.‎

This paper presents a high-sensitive envelope detector in 2.7GHz to 3.9GHz frequency band using 180 nm CMOS technology. The detector is based on the Successive Detection Logarithmic Amplifiers (SDLA) and consists of three sections: the detector core (rectifier), semi-logarithmic RF amplifier, and output stage. The detector core is based on the unbalanced source coupled method, which provides a full-wave rectifier. The frequency bandwidth and transition time of detector core are 0.1GHz to 10 GHz and 1ns, respectively. In this paper, the sensitivity of the detector is also improved about 2 dB by injecting part of the input signal into the tail current path. To amplify the signal, a proposed low-noise amplifier(LNA) with a single input and differential output and a simple differential amplifier in series are used. The output stage provides a low pass filter and drives a 2pF output capacitance, without increasing the transition time. By using RF power limiters in the outputs of LNA and differential amplifier, a semi-logarithmic behavior for the total circuit is obtained while preventing saturation of amplifiers due to the large input signal. Post-layout simulation results by Spectre-RF show the sensitivity of -45dBm and the rise and fall times of less than 1.2ns, which is a significant improvement compared to recent reported work. The semi-logarithmic dynamic range and the power consumption of this detector are 20dB and 12mW from a voltage source of 1.8V, respectively. The occupied area of the detector core is only 72µm×72µm, while the active area of the total detector, including amplifiers, limiters, rectifiers and output buffer is 0.7mm×0.55mm.

One of the most important parts of communication systems are matching networks. If the matching networks are not broadband, no matter how good the rest of the communication system is, the system’s bandwidth is bounded by matching network. In this project, we explain the real frequency technique and use it to design and simulate a broadband matching network for use in a broadband power amplifier. The designed prototype operates from 0.1 up to 4 GHz with around 12 dB power gain and delivers approximately 40 dBm or 10 Watts of power. The power added efficiency of the device more than 47% in the wide operation bandwidth. CGH40010F GaN HEMT device of Wolfspeed-Cree Company is used in the design.

In this paper, the transmitted signal of multi-antenna radars with the desired covariance matrix is designed to be implemented using a fully correlated base signal (similar to phased-array radar) and using space-time codes with much lower complexity and cost. Using the proposed model, the transmitted signal will depend on the contents of the space-time codes as well as the transmitted base signal. By selecting a completely correlated and identical base signal for all transmitted antennas, the transmitted signal covariance matrix will depend only on the contents of the space-time codes. Therefore, by designing the space-time code, the desired covariance matrix is achieved. The result of this method is the achievement of any desired covariance matrix in the transmitter (and consequently the optimal performance, such as the desired beam pattern or high probability in target detection) using the very simple structure of phased-array radar as well as space-time codes. This method does not require several types of signal generators with different specifications (waveform variation). The receiver filter is also designed in accordance with the existing conditions in such a way that the best performance of the target detection probability be achieved.

In this research we are looking for finding a way to suppress radars of an integrated air defense system with the help of a group of cooperative UAVs using noise jamming. The purpose of this research is to propose a target allocation and path planning algorithm so that they can fly closer to their target without the risk of being detected. Objective of optimization is to reduce usage of resources including operation time, increase noise level in radars’ receivers and reduce probability of UAVs being detected by search radars. probability of detection depends on relative position and attitude of UAVs and each radar and the number of UAVs jamming each radar. To simplify the computational complexity of solving the path planning problem on a continuous domain, we discretize airspace and use a honeycomb sampling method instead of a uniform sampling method. This discretization reduces the path planning to a constrained shortest path problem over a finite graph. In this method we sample more points in regions where the optimal path is more likely to deviate from a straight-line. The target allocation problem too can be turned into a finite graph and solved using pulse algorithm. Eventually with the help of Computer simulation, the effectiveness of the proposed algorithms is examined in a typical mission of cooperative UAVs jamming an integrated air defense system.

Receiving more accurate information from an area in SAR images requires high-resolution imaging of that area. In order to achieve high resolution SAR images, spotlight imaging mode is used. Two-step focusing approach, is an efficient approach to eliminate doppler spectrum folding in spotlight mode and focus raw radar data. Relevant articles and references, have only used fast fourier transform and complex coefficient to eliminate data spectrum folding in implementing this approach. But in fact, eliminating the doppler spectrum folding in this approach, requires a series of correction coefficients in addition to fast fourier transform and complex coefficients, which in none of relevant references and articles, are not provided. In reality, the relevant references have only provided a general method of implementation of this approach and have omitted to provide these correction coefficients. Failure to consider these correction coefficients in the implementation of this approach, will cause distortions in the compressed signal. In this paper, the desired correction coefficients are extracted and the block diagram of signal processing is modified by using these coefficients. Also, simulations have been performed that shown the distortion due to not considering the correction coefficients. Implementing two-step focusing approach with considering correction coefficients is the main topic of this paper.

In recent years, intelligent structures play the key role in different military fields and take the place of human operators in novel armed services step by step. Threat evaluation of flying air targets in command and control systems is performed by expert operators through their knowledge and experience. Analysis of input data in the data-fusion systems is a very difficult task requiring complicated decision. Capability and accuracy of intelligent systems for threat prediction of flying air targets base on different received parameters can be a great assist in final macking decision. In this study, a neural network and ANFIS regression models are used to determine the priority of the threat of flying air targets in the command and control systems intelligently and instantly. The error of trained neural network and ANFIS for test dataset are 4.14% and 1.65%, respectively indicating superior ability of these structures in threat estimation of flying air targets. Furthermore, relationships among target variables and threat level is studied. Finally, a dynamic battle scene with different flying air targets is simulated and developed moled is validated.

This paper proposes a new method for the design of a broadband dual-beam reflectarray antenna with a single feed. This method is based on the producing a null in the antenna radiation pattern and the appearance of two beams. The reflection phase distribution on the aperture of the antenna has been studied to achieve this null. A single layer subwavelength element with multi-resonances is used as the phase shifting unit cell due to its wide linear phase range. A comprehensive theoretical analysis and full-wave simulation are accomplished and the results are in good agreement. Two dual beam reflectarray antennas operating at X band are designed using the proposed method and the geometrical method. The improvement in the gain bandwidth and the beam squint of the designed reflectarray using our proposed method is observed compared to the other one. The simulation results show the improvement of 1.1% in 1-dB gain bandwidth and in the beam squint for the proposed design method compared with the other one.

Design of sparse array antenna that can create the desired radiation patterns with minimum number of elements, is a favorite research area. The synthesis sparse array problem can be modeled with appropriate constraints on the number of solve space members, namely l_0-norm of the weight elements. But it is a non-convex problem that requires to solving a NP-hard problem. An interesting ideas is mentioned to relax problem to convex problem. The proposed solution is based l_1-norm; The algorithm used here, first determines the optimal radiation pattern with convex optimization. then by using iterative weighting l_1-norm, sparse array is obtained by removing those elements that weights of them are almost zero and optimally determines the position of the element. As a result, by solving the non-convexity property of the problem, the optimal solution is provided with a reasonable computational time. The purpose of the optimization method is to minimize the number of elements, observe the constraints related to the requirements of the radiation pattern and reduce the calculation time. This research, in its case study, was able to sparse the 11×11 array (121 elements) to 42 elements (increase PSL) and 37 elements (increase mainlobe beamwidth) by adjusting the relevant parameters such as DRR, γ and ε.

Phase velocity and interaction impedance measurements on slow wave structures are especially important in the microwave tubes industry. Among the methods available for measuring the phase velocity and interaction impedance, non-resonant perturbation method is simple yet accurate and applicable. In this paper, the correctness of this method for helical slow wave structure is investigated with simulation and analytical methods and based on it, a device is built that has the precision needed to move the perturber inside a slow wave structure. The results and measurement capabilities of this device have been demonstrated using simulations in CST software and analytical relationships.

Due to developments have been occurred in ECCM techniques of radars, pulse separation methods in ESM systems rely on intera-pulse modulation instead of analyzing pulse descriptive words in traditional methods. Extraction of pulse modulation is a suitable method but in the case of multi-mode radars, the number of targets is overestimated by changing the intra-pulse modulation. The purpose of this paper is to detect multi-mode radars with various types of internal modulation in a dense radar environment. The proposed solution is to add multi-mode radars detection to the existing pulse separation methods at the post processing stage. This method involves providing an appropriate framework for examining the separated pulse string by defining and selecting similarity criteria from the extended PDW. In this method, at first the distinguishing features of each radar are extracted and the similarity criteria of each feature are calculated to check the similarity between the two pulse strings. Input data contains information separated from real radars received by ESM system. Due to the uncertainties of each criteria, similarity scores are computed through the fuzzy roles and normalization and training dataset is formed. The data table is then used to train a perceptron neural network. The trained network can detect multimode radars automatically. To test the network, a section of the data table is applied to the network and trained network succeed in 100% of test data to distinguish multi-mode radars from the distinct radars.

Radial line slot array (RLSA) antennas are a type of high-efficiency plane array antenna used to generate circular polarization. In these antennas, the second-floor wave-guide is filled with a dielectric material to eliminate the grating lobes caused by the array of structures. All input power to the antenna must pass through a confined space that includes dielectric, air, and metal. This space has corners where the intensity of the electric field will be very high and, therefore, at high powers, will reduce the power transmission capacity of the antenna. For this reason, instead of using a dielectric in the space between two wave-guides, a slow radial wave structure is used. In this paper, first, a mathematical analysis of slow-wave structure is performed based on the Fluke theory. Next, a slow-wave structure is designed, simulated, and optimized for use in a 10.2 GHz Radial line slot array antenna.The simulation results showed that the minimum reflection coefficient is -20 dB, and its bandwidth is 251 MHz. Finally, a prototype of the simulated antenna was constructed using the proposed structure. Antenna power analysis showed that the minimum tolerable antenna power is 625 MW.